1. Field Of The Invention
This invention relates to thermally controlled persistent current switches for superconductive circuits, and more particularly to a construction of a switch pack having several such switches with each switch employing a cupro-nickel matrix superconductor and being capable of simultaneously switching a number of coils.
2. Discussion Of The Prior Art
Switches for turning superconductive magnet circuits on and off are well known. They typically comprise a length of superconductive wire and a heater element. The switch is cooled to a temperature well below the critical temperature of the superconductor (e.g., 9.degree. Kelvin) by immersing it in a cryogen such as liquid helium. At or below the critical temperature, the resistance of the wire falls to zero, thereby making the wire superconductive. In the superconductive or persistent state, the switch is "on" because it has no resistance. The switch is turned off by turning the heater element on, which raises the temperature of the superconductive wire above the critical temperature, thereby producing a finite resistance to the flow of current.
Switches of this type are often used in circuits which include a superconductive magnet coil, such as in a magnetic resonance (MR) scanner. There may be many superconductive magnet coils, each having its own switch, which cooperate to produce a desired magnetic field. As such they are inductively coupled, i.e. the portion of the magnetic field which each contributes is affected by the magnetic fields which the other magnets produce. Changing the current through one of the coils changes the magnetic field it produces, which changes the current through any other coils inductively coupled with that coil, which changes their magnetic fields, which changes the current through the first coil, etc. Thus, inductive coupling creates a problem in adjusting the current through each magnetic coil because of their interrelationships. Also, individually adjusting each coil would result in a relatively large boil-off of liquid helium because of the time and number of changes in current which would be required.
This problem is overcome by switching all the inductively coupled magnet coils substantially simultaneously. During charging, each switch is connected in parallel with the corresponding magnet coil and a power supply for the particular switch which is adjustable to ramp up to the desired current through the coil at the same time that the desired currents through all the other coils are reached. During ramping, the switches are off. At the time the desired currents are reached, all of the switches are turned on at substantially the same time to conduct the persistent current and the power supplies are disconnected.
The coils of superconductive switches have been made up of whatever length of superconductive wire is necessary to achieve the desired resistance in the "off" state and provide a great enough heat capacitance in the switch to avoid damage when the switch is off. In prior switches which have employed copper matrix superconductor, a relatively long wire has been required to achieve even a relatively small resistance. Because of the small resistance, the switch also had to be able to absorb large amounts of energy. For example, in one such switch, 280 feet of copper matrix superconductor was required to achieve a resistance of approximately 0.05 .OMEGA. at 10.degree.-20.degree. K. A higher resistance is desirable because it allows less energy absorption by the switch when it is off, thereby reducing the boil-off of the cryogen during charging and discharging of the superconducting circuit.
Another consideration in superconductive switches is current carrying capacity. Copper matrix switches have generally been capable of high current densities, but at the expense of the relatively poor "off" characteristics referred to above.
Cupro-nickel matrix superconductors are known to have higher "off" resistance than copper matrix superconductors. However, cupro-nickel matrix superconductors are inherently extremely unstable. Slight movements in a magnetic field can cause them to "quench", losing their ability to conduct at zero resistance.